This continuing investigation is concerned with identifying cerebral mechanisms accounting for the structure and production of the isolation call. This vocalization, which appears to be common to all mammals, is perhaps the most primitive and basic mammalian vocalization, serving originally to maintain maternal-offspring contact. In a comparative approach to this problem we are utilizing squirrel monkeys (Saimiri sciureus) which have a typical isolation call that lends itself to experimental analysis. This year's findings have helped to delineate further the core gray matter and adjacent tegmental structures at the junction of the thalamus and midbrain that appear to be essential for the patterning and production of the isolation call. This region is known to receive projections from the medial frontal limbic cortex. Our experiments thus far indicate that ablation of the entire strip of limbic cortex extending from the anterior cingulate down to, and including, the posterior part of the gyrus rectus, permanently interferes with the spontaneous production of the isolation call. Incomplete ablation of this strip or aspiration of the medial frontal neocortex at the level of the genu results in only a temporary elimination of spontaneous calls. Finally, in regard to vocal effector mechanisms, it is notable that bilateral destruction of the lateral central gray and adjoining tegmental structures at the caudal level of the inferior colliculus eliminated all the recognized calls of squirrel monkeys except cackles. The region in question is believed to encompass an effector pathway leading to Nucleus ambiguus.